On 1/17/12 10:25 AM, Ken wrote:
> On Jan 17, 2012, at 10:47 AM, Jim Lux wrote:
>> What does "effective" mean?
>> If you define it as "getting RF power at the feedpoint launched into space",
>> ANY size lossless antenna works pretty much the same.
> I have trouble accepting that. We will have to agree to disagree.
that's just basic electromagnetic theory. If you have accelerating
charges, some energy is radiated away. All the antenna does is provide
a place for those charges to move in a desirable fashion.
You can start with an idealized half wavelength and calculate how much
charge is moving and changing speed for all points and from that figure
out how much energy radiates away. Then, you can shorten the antenna,
and redo the calculations, and still, there's no where else for the
energy to go..
Another way to look at it is this: Power supplied to the feedpoint
either stays as energy "stored in the antenna" or "radiated away".
AFter an initial transient phase, when you reach steady state, the power
supplied to the antenna exactly matches the power radiated away (because
there's no loss in the antenna). A physically small antenna will have
an enormous amount of energy stored in it: either in the magnetic
field/current or in the electrostatic field/voltage. (and actually, in
any antenna, energy is constantly flowing back and forth between the
electric and magnetic fields)
And therein lies the rub for "practical" antennas. if the energy stored
in the antenna is, say, 100 times that radiated away, and there's ANY
loss in the system, each time that energy circulates back between
magnetic and electric fields, you lose a little bit.
Think of the idealized antenna as a lossless LC resonant tank with a
small resistance in the circuit. The small resistance is the "radiation
resistance", and it gets smaller as the antenna gets smaller. Meanwhile
the reactance is still there, storing energy (and interestingly, for
small antennas, the reactance gets bigger as the antenna gets smaller).
So the ratio between reactance and resistance gets big(called Q in
tuned circuits.. but really it's the ratio between stored/lost energy)
An infinitely small antenna has infinitely small radation resistance, so
the Q is infinitely large.
But, by definition reactance is lossless, so the overall energy balance
has to be kept: all the energy in gets still gets radiated away. (and
not even isotropically.. it has almost 2dB directivity)
the other issue with "practical" antennas is that you generally don't
have an idealized source which can put appreciable power into something
with a very unusual feedpoint impedance. So you add some lumped
components (hopefully low loss) to try and transform what you got on the
antenna to what you need at the source.
As an interesting experiment, think about what kind of antenna would be
optimum to directly connect to a FET amplifier (which has an output Z of
a few ohms and is almost a pure constant voltage source)
Likewise, what would be optimum to connect to a high impedance constant
You can get all kinds of interesting systems with this sort of thing.
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